Previous studies which have examined the effects of lesions of the habenula on active avoidance learning have not provided unambiguous support for response deficits. Moreover, interpretation of early studies is confounded by large lesions which damaged adjacent structures. We report the effects of smaller circumscribed lesions of the habenula complex on a simple one-way active avoidance paradigm in three separate experiments in which the required operant was a step or jump onto an elevated platform. In the first study involving avoidance of shock of low intensity (0.5 mA) with an average long intertrial interval of 8 minutes, lesioned animals were not significantly different from sham operated controls. However, in following experiments in which stress levels were increased by raising the shock intensity and reducing the intertrial-interval, or in which the operant was made considerably more demanding by raising the height the animals had to jump to make an avoidance response, there were large lesion induced deficits in avoidance responding. The absence of significant differences between lesioned and sham operated controls in escape latencies suggested no lesion induced impairment of the response to the shock. It is suggested that the data not only support an effect of habenular lesions on active avoidance learning but also are consistent with a previous suggestion that the functional effects of lesion of the habenula on behaviour are effected through changes in dopaminergic function.